What are the health risks that a high-protein diet may pose? What should we watch out for and what should we know?
This article will be the first in a series, which will focus on protein consumption.
This one reviews the effects of protein on health.
PROTEIN AND BONE HEALTH
A seemingly negative aspect of higher protein intake is related to bone health and the potential increase in dietary acid load from it.
The acid ash hypothesis posits that increased protein intake (specifically sulphur-containing amino acids) leads to a greater dietary acid load, which is regulated through the release of bone-derived alkalizing compounds, resulting in bone resorption, osteoporosis and hypercalciuria.
However, in this study, bone mineral density was not correlated with the ratio of animal to plant protein intake, despite the fact that animal protein provides acid precursors, while vegetables provide base precursors not found in animal foods.
Worse still, the so-called acid ash hypothesis has been examined in meta-analyses and was found to lack support.
A meta-analysis in 2009 found that, in contrast to the acid ash hypothesis, higher phosphate intake was associated with decreased urinary calcium and increased calcium retention.
In this analysis which used the Bradford/Hill criteria to test causality, it was clear that the association between dietary acid load and osteoporotic bone disease was not supported by evidence, while in a recent analysis conducted by the US National Osteoporosis Foundation, the authors concluded that there are positive rather than negative trends in bone density in most areas of the bone when on a high-protein diet. In agreement with this conclusion, it has been suggested that dietary protein supports bone health, but this is probably only the case when calcium intake is adequate.
In a 6-month intervention in women with exercise experience, comparing the effects of a high-protein diet (2.8g/kg per day) versus a control group (1.5g/kg per day), the researchers found no differences in bone density, while in a weight loss program with diet and exercise that lasted 16 weeks, consumption of dairy foods and higher protein, combined with adequate intake of calcium and vitamin D resulted in in improved bone health and metabolic health indicators in overweight and obese young women.
Besides, it has been pointed out that, the correlation between changes in calcium excretion and changes in urinary acid excretion is not evidence that the source of the excreted calcium is bone or that this calciuria contributes to the development of osteoporosis.
In fact, data from a dual stable calcium isotope study suggest that the main source of increased urinary calcium from a high-protein diet comes from increased dietary calcium absorption rather than bone resorption.
Therefore, in general, it seems that protein not only does not harm bones, but it is an essential nutrient for bone health. Its intake seems to have a positive effect on calcium balance and therefore the maintenance of bone mineral density.
Since exercise provides a stimulus for protein growth in skeletal muscle, it is not uncommon for intake levels ranging from 1.4g/kg to 2.0g/kg per day during prepubertal development to be recommended, so that this stimulus can be converted into additional muscle tissue, which in turn is an important predictor of acquisition of bone mass during this period.
In addition, there is a lack of scientific evidence linking higher intakes of dietary protein to negative outcomes in healthy people who exercise, while some evidence suggest that older men and women (the part of the population most susceptible to osteoporosis) should consume protein above current recommendations (0.8g/kg per day) to optimise bone mass.
PROTEIN AND TYPE 2 DIABETES
The relationship between high protein intake and type 2 diabetes is still under debate and results vary depending on the duration of the study, the protein source and whether there was weight gain. The most important factor of concern linking protein to diabetes is amino acids.
Circulating levels of branched-chain amino acids (BCAAs) are excellent biomarkers of metabolic health and their elevated blood levels are markers of insulin resistance and consequently of type 2 diabetes.
Recent research has shown, however, that the causality is reverse, as not only the data do not support their role as causal, but that increased concentrations of amino acids are due to their defective catabolism compared to healthy individuals, something that is also supported by genetic studies.
So, we can say that when diets with high protein and controlled calorie intake are applied, with the aim of losing or maintaining weight, we can usually expect favourable results in relation to insulin sensitivity.
On the other hand, a diet that increases body fat will always have negative effects on metabolic health.
PROTEIN AND KIDNEY HEALTH
A common reason why higher protein intakes are generally not recommended for the elderly is the increased risk of kidney disease.
However, analyses show that this has no basis.
There is general agreement on the effect of dietary protein restriction on the attenuation of the decline in glomerular filtration rate (GFR) among most forms of renal injury in humans.
Although there are potential injurious pathways that have been proposed whereby diets higher in protein could facilitate renal injury, the magnitude of the effect and whether it exists remain unknown.
The concept that protein restriction can delay the deterioration of kidney function associated with aging was based on studies in rats in which low-calorie or low-protein diets slowed the development of chronic renal failure.
However, deterioration of kidney function quite differently in rats, which means that this mechanism is unlikely to work in humans.
Also, when creatinine clearance was measured in humans at intervals of 10 to 18 years, the deterioration was not associated with protein intake.
In fact, the correlation of creatinine clearance with protein intake shows positive gradient relationship, suggesting that low protein intake itself reduces renal function.
This meta-analysis showed that the glomerular filtration rate (GFR), as an indicator of renal function, does not decrease when individuals have higher protein intake, but rather increases, even though this meta-analysis in individuals with normal renal function suggests that the effect may be insignificant or even non-existent, even in populations at higher risk for renal impairment, such as people with type 2 diabetes.
It is believed that this overfiltration of the kidneys is the result of nephron loss. However, in humans, glomerular hyperfiltration occurs at the whole-kidney level as a result of increased blood flow to the kidneys.
It is important to understand that the ability of the kidney to increase GFR under certain stimuli or demands is a normal response to increased solute load and does not represent a risk factor for the development of chronic kidney disease. In addition, pregnancy and unilateral nephrectomy are characterized by significant renal hypertrophy and hyperfiltration with increased GFR, while renal function remains normal.
These factors lead to the conclusion that dietary protein content is not responsible for the gradual decline in renal function over time, but rather that this reduction in glomerular filtration rate is a natural consequence of reduced protein intake as we age.
In the most recent round of discussions on Dietary Reference Intakes, the US Institute of Medicine concluded that the protein content of the diet is not responsible for the gradual decline in kidney function over time, with the report of the World Health Organization and the International Food and Agriculture Organization reaching the same conclusion.
Thus, the question arises whether and how and for how long these high protein intakes can adversely affect health.
In one of the longest-running dietary intervention trials, which lasted 2 years, in which higher to lower protein intakes were compared, it was reported that in otherwise healthy obese individuals, a low-carbohydrate but high-protein weight loss diet was not associated with harmful effects on GFR or proteinuria.
This study in resistance-trained men, lasted one year. Subjects were alternated between their usual diet (which was already high in protein) and a higher protein diet. Thus, on average, each person consumed their usual diet for 6 months and a higher protein diet for 6 months. Throughout the course of the year, they consumed 2.51-3.32g/kg of protein per day. However, there were no detrimental effects on blood lipid measurements, as well as liver and kidney function, despite the overall increase in energy intake during the high-protein phase.
Similarly, overweight and obese volunteers with pre-diabetes on a higher protein diet had a significant increase in serum urea and urinary urea/creatinine ratio after one year. However, there were no correlations between increased protein intake and creatinine clearance, estimated glomerular filtration rate, urinary albumin/creatinine ratio or blood creatinine. Thus, after 1 year of higher protein intake in pre-diabetic older adults, no evidence of impaired kidney function was found.
In addition, this investigation showed that on a weight loss diet, both a medium and a high protein diet lead to normalized renal function over a 12-month period in people with type 2 diabetes and early kidney disease.
However, the limits of adaptation to a long-term high-protein diet need to be explored.
Although it appears that dietary protein intakes above the Reference Dietary Intake are not harmful to the healthy exercising population, people vulnerable to diseases of the kidney (such as kidney stones) or with mild renal impairment should monitor their protein intake closely, as epidemiological studies provide data suggesting that protein intake may be associated with the progression of renal disease, as in studies such as this one, where only patients with pre-existing dysfunction appeared to have an increased risk of developing kidney stones and kidney disease.
Truth is that more research needs to be conducted in adults and the elderly in relation to exercise, skeletal muscle hypertrophy and protein intake and their cumulative effects on health in general.
Overall, the implementation of a high-protein diet does not appear to have adverse effects on already healthy individuals. Although no clear recommendation exists that defines the safe upper limit of protein intake, consumption of up to 40% of total calories from protein daily has not been associated with increased risks on health in otherwise healthy people.
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